Transmission system



Dec. 2, 1941. R. H. HERRICK TRANSMISSION SYSTEM Filed Aug. 14, 1939 IIIL INVENTOR. ROSWELL H HERRICK l BY ma, xwwim ATTORNEYS Patented Dec. 2, 1941 UNETED STATES PATENT 9F F E 14 Claims;

The present invention relates generally to improvements in two-way signal current transmission systems of the type in which signal controlled switching circuits are provided for par-- tially or completely blocking, under certain conditions, certain of the signal current channels included therein, and more particularly to improvements in telephone substation circuits having incorporated coupled signal current channels.

for the transmission of incoming and outgoing; signal currents.

When separate signal current transmission channels are incorporated in .a substationicirc'uit.

for transmitting outgoing and incoming signal currents between the associated transmitting and receiving means and the line extending thereto,

the channels must necessarily be electrically and.

acoustically coupled at the respective ends thereof, thus effectively forming a closed loop over which side tone currents -may be transmittedto the receiving means during operation of the transmitting means and vice versa. Moreover, if signal current amplification is introduced'inone or both of the channels so that the. energy gain around the closed loop exceeds'the. energy lossestherearound, oscillation oi. the system may occur to produce interference commonly known as singing. In order t obviate these difiiculties; various arrangements have been usedfor. par tially or completely blocking eachsignal channel when not in use or, alternatively, for introducing losses in each channel, whennot in use, sufficient to prevent singing and substantially to reduce the flow of side tone currents.v 'Such arrangements must necessarily be so designed that speech clipping at the beginning and end'of each signaling interval is prevented.

It is an object of the present invention, therefore, to provide telephone substation apparatus ofthe character described WhGIBiIIithe'BlilIIillhtion of singing and side tone; suppression: are accomplished in a positive, reliable'andsimproved manner and wherein speech clipping: isisubstan tially prevented.

Itis a further object of the invention to p'r-ovide improved substation apparatus'o'f thecharacte'r described wherein a single sound" translating device is arranged to be used asboth the signal transmitting means and as the signal receiving means.

It is another object of the invention to provide an improved two-way signal currentltransmission system of the character described wherein the elimination of singing and side tone-suppression are accomplished'in a positive and reliable-manher with a minimum of circuit apparatus;

In the'illustrated embodiment-of the invention there isprovided a telephone substation circuit which is adapted to be connected to a line -ahd comprises a sound' translating device whichis arranged to serve both" assignal transmitting and signal receiving means. This device is-c0u-' pled'to the associated line through a signal transmission system which: comprises a pair'of channels for transmitting; signal currents between the line and the device in opposite directions. The transmission system' has'incorporate'd were:

in" signal current" controlled-switching meansfor' selectively controlling the signal current trans mission efiiciencies of the respective channels" in accordance with the direction of signal current flow-therethr'ough. More particularly, theztran's mission system comprises a pair of vacuum tube amplifiers which are respectively included in the two signal current channels, the amplifier inserted'" in one of thechan'nels having'co'nnected in tandem therewith a'voltage' reducing circuit. This circuit, in cooperation with the amplifier included 'in. theother channel, functions to con-. trol'the switching means in a mannersuch that the other channel, which is normally blocked or inactive, is rendered active-and the channel in which the voltage reducing circuit is incliide'd is rendered inactive in response to signal currents incoming to the substation over the as'soc'i ated line. To this end there is provided a control circuit which is coupled to e'ach of thetwo'chamnels, .rneans'l being embodied therein for providing two control voltages in response to signal currentstraversing the transmission system. The arrangement is such that the voltage. reducing circuit. mentioned above and the amplifieraincludedfin the other ch'annel function tounbalance the two control voltages only when signal. cur.-

rents traverse the said other channel. These two control voltages are impressed across the windings of a difierentially wound control device. whereby the control device is caused to operate only'when the control voltages are unbalanced. This control device is arranged to control. the operationof a set of switching contacts which are provided for controlling the signal current transmission efiiciencies of'the two channels;-

Further features-of the invention'pertain. to the particular arrangement of the circuit -elements wherebythe above-mentioned andadditional operating features are'attained: The novel features believed to be characteristic of: the: invention are-set-forth with-particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawing, the single figure of which illustrates a telephone substation circuit having incorporated therein the features of the invention as briefly outlined above.

Referring to the drawing, the substation circuit there illustrated comprises a sound translating device I coupled to a line extending to the bracketed terminals II by means of a pair of signal current transmission channels I2 and I3, the channel |3 being utilized to transmit incoming signal currents from the line to the sound translating device l0 and the channel I2 being utilized to transmit outgoing signal currents from the sound translating device ID to the line terminals II. The sound translating device I0 is preferably in the form of a loud speaker of the moving coil type having embodied therein a signal current coil I4 which is arranged selectively to be coupled to the input circuit of a vacuum tube amplifier I5 included in the channel |2 or the output circuit of a vacuum tube amplifier I6 included in the channel I3 through the operation of switching contacts controlled by a relay 20 which is included in a control circuit 25. More particularly, the amplifier l5 comprises a vacuum tube 3| of the pentode type having input electrodes 32 and 33 normally coupled to the loud speaker Ill through a transformer 34 and output electrodes 33 and 35 included in an output circuit which comprises the primary winding of a coupling transformer 36 and a source of anode current 31. For the purpose of providing a bias voltage between the input electrodes 32 and 33, there is provided a cathode biasing circuit which includes a resistor 38 shunted by a signal current by-passing condenser 39. The amplifier I6 included in the signal current transmission channel I3 similarly and 50 which are connected in parallel across the source 31. Preferably the discharge tubes 41 and 48 are in the form of neon lamps. A lamp of this type possesses the characteristic of being nonconductive until a predetermined voltage commonly known as the break down voltage is impressed across the terminals thereof, at which time the lamp is rendered conductive and the resistance thereof is reduced from an exceedingly high value to a relatively small value. After a lamp of this character is rendered conductive, the voltage across the terminals thereof must be reduced to a value somewhat less than the break down value before the discharge through the lamp is extinguished. The discharge device 41 is connected across the upper adjustable portion of the potentiometer 49 in series with the winding 2| of the relay and a resistor 5|. The

I setting of the potentiometer 49 is such that the voltage impressed across the terminals of the discharge device 41 over the circuit just described is of less magnitude than the voltage required to render the device conductive or to sustain a discharge through the tube. For the purpose of increasing the voltage across the terminals of the discharge device 41 when outgoing signal currents traverse the transmission channel l2, this device is coupled to the output circuit of the amplifier I5 over a path including a condenser 52 and a current limiting resistor 53. With this arrangement the tube 41 is rendered conductive during alternate half cycles of signal currents traversing the output circuit of the amplifier I5 Y so that a pulsing direct current is caused to travcomprises a vacuum tube 40 of the well-known pentode type having input electrodes 4| and 42' coupled to the line terminals ll through a coupling transformer 43 and output electrodes 4| and 44 arranged to be coupled to the moving coil M of the speaker Ill through a coupling transformer 45. A cathode biasing circuit comprising a resistor 46' shunted by a signal current by-passing condenser 41' is provided for biasing the control grid 42 to the correct negative potential with respect to the cathode 4|.

Anode current is supplied to the anode 44 of the tube 4|] from the source 31 through the primary winding of the transformer 45. As illustrated in the drawing, the screen grids respectively embodied in the tubes 3| and 40 are connected directly to the positive terminal of the source 31. Preferably these tubes are of the character commercially known as the Type 4'1 tube having incorporated therein cathodes of the filamentary erse the winding 2| of the relay 2%.

In order to smooth out the fluctuations of this current, a smoothing condenser 54 is provided which is connected in shunt with the series connected resistor 5| and winding 2|. The discharge device 48 is similarily connected across the lower adjustable portion of the potentiometer 50 in series with a resistor 55 and the winding 22 of the relay 20, the potentiometer 50 being so adjusted that the voltage impressed across the terminals of the device 48 is of a slightly less magnitude than the voltage required to render this device conductive or to sustain a discharge therethrough. In order to render the discharge tube 48 conductive during alternate half cycles of signal currents traversing the incoming signal current channel I3, this device is coupled to the output circuit of the amplifier I6 over a path comprising a condenser 56 and a current limiting resistor 51. Fluctuations of the pulsating direct current traversing the winding 22 of the relay 20 when the tube 48 is conductive are reduced through the provision of a smoothing condenser 58 connected in shunt with the series connected winding 22 and resistor 55.

The switching elements controlled by the relay 20 comprise a pair of normally closed contacts 23 and 24 arranged to short-circuit the secondary winding of the transformer 45, and an armature type to insure quick heating when energized from a suitable alternating current source. The cathodes 33 and 4| of the two tubes are connected in parallel across the secondary winding of a voltage dropping transformer 46 having its primary winding arranged to be connected to any available source of commercial frequency current.

The switching control circuit 25 comprises, in addition to the relay 20, a pair of gaseous discharge tubes 41 and 48 and a pair of associated potentiometer or voltage dividing resistors 49 26 which in cooperation with the contact 24 is arranged to open the indicated short-circuiting path and to close a path for short-circuiting the primary winding of the coupling transformer 34. An additional armature 21 is provided for shortcircuiting the winding 2| of the relay 2|] when this relay is operated, thereby to render the relay slow to release in a well-known manner.

In order to assist in the unbalancing of the voltages impressed across the relay windings 2| and 22 of the control circuit 25 when signal currents traverse the transmission system in one diof side tone currents back to the moving coil l4 of the loud speaker l cannot occur.

With the transmission system conditioned for operation, signal currents incoming thereto over the associated line are caused to traverse the primary winding of the coupling transformer 43 and the secondary winding of the transformer 36 in the manner previously explained. Due to the voltage step-down action of the circuit 60 and the voltage reducing operation of the transformer 36, the signal voltage appearing across the channel l2 in the output circuit of the amplifier I5 is quite small. Accordingly, the potential fluctuations of the lower or control terminal of the discharge tube 41 are of such small magnitude that this tube is not rendered conductive. The amplified signal voltage developed across the output circuit of the amplifier I6 is, however, of sufiicient magnitude to cause the break down of the discharge tube 48 during alternate half cycles of the signal voltage. Accordingly, only the winding 22: of the relay 2!] is energized in response to signal currents incoming to the substation circuit. When this winding is first energized the relay 20 immediately operates to short-circuit, at its armature 2?, the winding 2|, thereby to render itself slow to release and to restrict the flow of transient currents through the winding 22, which currents might be reproduced as clicks by the loud speaker I0. Upon operating, the relay 2!! also completes, at its armature 26 and the associated contact 24, an obvious path for shortcircuiting the primary winding of the transformer 34, thereby to block or render inoperative the signal transmission channel [2. At the contact springs 23 and 24, the relay 2!] opens the path normally short-circuiting the secondary winding of the transformer 45, whereby the incoming signal currents may be transmitted through this transformer to the moving coil l4 of the loud speaker ID to be reproduced as sound waves in the usual manner. It will be noted that with the relay 20 operated the transmission channel l2 is completely inoperative. Accordingly, local oscillation of the system is prevented and complete side tone suppression is achieved. With this arrangement it is impossible for a user of the illustrated substation apparatus to break in upon the speech train being received over the channel l3. With the winding 2| of the relay 2!} short-circuited the release time of this relay' is sufiiciently long to prevent the restoration of the relay during the short intervals between syllables of speech. Thus, hang-over periods are provided so that voice currents received substantially without interruption serve to maintain the transmission channel i2 inactive and the channel I3 active.

When the incoming signal currents are interrupted for a time interval exceeding the release period of the relay 20, this relay restores to reestablish, at the contact springs 23 and 24, the path short-circuiting the secondary winding of the transformer 45 and to interrupt, at the armature 26 and the contact spring 24, the path shortcircuiting the primary winding of the transformer 34. Thus, the channel I3 is blocked or renderedinactive and the channel i2 is reconditioned to transmit outgoing signal currents to the associated line. Upon restoring, the relay 20 also opens, at its armature 21, the path shortcircuiting its winding 2 I, thereby to condition itself for fast operation. When energized, this relayoperates with sufilcient rapidity to prevent any substantial clipping of the voice currents received at the beginning of a speech train and representing the first syllables of the train.

Although the invention has been described with reference to an arrangement wherein the transmission channels not in use are completely blocked, it is contemplated that a partial blocking of these channels during the inactive periods thereof may be used if desired. To this end, a resistor may be serially included in the channel blocking path, preferably by connecting the same between the contact spring 24 and the junction connection between the primary winding of the transformer 34 and the econdary winding of the transformer 45. With a resistor of proper resistance value connected in this manner, the coupling between the channels through the resistor may be held to a sufliciently low value substantially to prevent singing and to insure adequate side tone suppression, at the same time providing an arrangement whereby conversation break-ins may occur.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is contemplated to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A transmission system comprising a pair of channels for transmitting signal currents in different directions, and means comprising a voltage reducing circuit connected in one of said channels and amplifying means connected in the other of said channels for substantially changing the signalcurrent transmission efficiency of at least one of said channels.

2. A transmission system comprising a pair of channels for transmitting signal currents in different directions, circuit means operative to provide two balanced control voltages in response to signal currents traversing said system, means comprising a voltage reducing circuit connected in one of said channels for unbalancing said voltages only when signal currents traverse said system in one direction, and means controlled in accordance with the relative magnitudes of said control voltages for substantially changing the signal current transmission efliciency of at least one of said channels.

3. A transmission system comprising a pair of channels for transmitting signal currents in different directions, circuit means operative to provide two balanced control voltages in response to signal currents traversing said system, means comprising a voltage reducing circuit connected in one of said channels for unbalancing said voltages only when signal currents traverse said system in one direction, a relay including a pair of difierentially related windings across which said control voltages are respectively impressed, whereby said relay is normally in one circuit controlling position and operates to a second circuit controlling position in response to the unbalancing of said voltages, and means controlled by said relay for substantially changing the signal current transmission efficiency of at least one of said channels.

4. A transmission system comprising a pair of channels for transmitting signal currents in different directions, a signal current amplifier included in each of said channels, circuit means operative to provide two balanced control voltages in response to signal currents traversing said operas-11 system, means; comprising a voltage. reducing circuit connectediirrone'of said:- channels and the signal'tcurrentamplifierzincluded. in the other of said; channels for. 'unbalancing :said voltages only when" signaljcurrents traverse :said ystem in one direction, and means; controlled in. accordance with the: relative magnitudes-* of said control voltages: for. substantially changing the signal currentstransmissi'on"efficiency of at least one of said channels.

5. A transmission system comprising a pair of channels for transmitting signal currents in different directions, a signal current amplifier included in each ofisaid" channelscircuit means operative'to provide two balanced control voltages in responset'o signal'currents traversing said system, means comprising a voltagereducing circuit connected in. oneofsaid channels and the signal current amplifier included in the other of said channels for unbalancing saidvoltages only when signal currents traverses'aid system in one direction; arelay including. apair of difierentially related windings across which said-control voltages are respectively impressed, whereby said relay is normally in onecircuitcontrolling position and operates to a secondcircuit controlling position'in: response to the unbalancing of said voltages, andm ans controlled by said'relay for substantially changing the. signal currenttransmission efficiency of at least one of said channels.

6. A transmission'system. comprisinga pair of channels for'transmitting' signal currents in differentv directions, a signal current amplifier and a voltage reducingcir'cuit included in one of said channels, said amplifier and said circuit being connectedin' tandem in the order named in the direction of signal current transmission through said one channel, .a second signal current amplifierincluded in the other of said channels, a control circuit coupled to said one channel at a point between said first-named amplifier and-said voltage reducingcircuit and to said other channel at a; point following said second amplifier, said air-- cult including'means. for providing'two balanced control voltages in response to signal currents traversing said system, means comprising said voltage reducing circuit and? said second ampliher for. unbalan'cing said voltages when signal currents traverse said'other channel, and means operative in response to the unbalancing'of said voltages for substantially reducing the signal current transmission eiiiciency of said one channel.

7. A transmission system comprising a pair of channels for transmitting signal currents in difieren t directions, a signal current amplifier and a voltage reducing circuit included in one of said channels, said amplifierand said circuit being connected in tandem in the order named in the direction of signal current transmission through said one channel, a second signal current amplifier: included in the other oisaid channels and including an input circuit coupled through said voltage reducingcircuit to'the out-put circuit of said first-mentioned amplifier, whereby signal currents traversing said onechannel are transmittedtlirough said second amplifier, a control circuit coupled to said one channel at a point between said first-named amplifier and said voltage reducing-circuit and to said other channel at a point following said second amplifier, said-cirersei said; other. channel, switching means, norma-llyefiective: substantially to' reduceztl'ie signal current. transmission efiici'en'cy. of said: other channel, and means operative. in response to the unbalancing. of'said' voltages for'causing said switching.- mean's tov increase the signal current transmission efliciency of said other channel and substantiallyto reduce thesignal current transmission eilicienc-y of said Zone channel.

8..In a transmission system. including a. pair ef channels :fcr transmitting signal currents in diiferentldirections; anetwork for controlling the signal current transmission efficiency of at least one: of said cha nnels, said network-comprising, in combination; a-pair ofsdischarge devicea each of said devi'ces being nonconductive until voltage ci a predetermined magnitude is impressed across the terminals thereof, circuit m'eansfor impressing'signal voltages appearing across'one of' said channels upon one of said devices and for impressing signal voltages appearing across the other of'said cha-nnel's upon the other of "said'devices; thereby to render said devices; conductive, a pair of'control circuits respectively including said dischargedevices and each energized only when=the discharge device included therein is rendered conductive, a control device including'a pairof differentially related windings respectively included in said control circuits, whereby said control-device is-caused to operate from one position to another when only one of said discharge devices is rendered" conductive, and switchingmeans controlled by said control device for controlling the signal current transmission einciency' of at least one of saidchannels.

9 In a transmission system including a pair of channels for transmitting signal currents in different directions, means coupling said channelstcgether to provide substantially equal signalvoltagesacross predetermined points of said channels in response to signal currents traversingsaidsystem in one "direction and substantially unequal signal voltages across said predetermi'nedpoints of said'channel-sin response to signal. currentstraversing saidsystem in the other direction, a circuit network comprising a pair of discharge devices; each ofsaid devices being normally nonconductive and being characterized bya: predetermined break down voltage at which it--is* rendered conductive, circuits respectively including said devices and also'including means for" impressing biasing voltages across said. discharge devices which arelessinmagnitude than said break down voltage, circuit means for impressing signal voltages appearing across said predetermined point of one of said channels upon one of said discharge devices, and for impressing signal voltages appearing across said predetermined point of theother of said channels upon the other of said" discharge devices, whereby both of said" discharge devices are rendered conductive duringalternate half'cycles of said signal voltages when signal currents traverse said system in said one direction and only one of said dis-'- charge devices is rendered conductive when signal currents traverse said system'in said other directiom and switching means controlled in accordance with the 'currentstraversing said devices for controlling the-- signal current transmission e'fiiciency of at least one of said channels.

1'0. In a transmission system including a. pair of channels for transmitting signal currents in different directions, means coupling: said channels together to provide substantially equal signal voltages acrosspredetermined points off said channels in response to signal currents traversing said system in one direction and substantially unequal signal voltages across said predetermined points of said ch'annels in response to signal currents traversing said system in the other direction, a circuit network comprising a pair of discharge devices, each of said devices being normally nonconductive and being characterized by a predetermined break down voltage at which it is rendered conductive, circuits respectively including said devices and also including means for impressing biasing voltages across said discharge devices which are less in magnitude than said break down voltage, circuit means for impressing signal voltages appearing across said predetermined point of one of said channels upon one of said discharge devices, and for impressing signal voltages appearing across said predetermined point of the other of said channels upon the other of said disch'arge devices, whereby both of said discharge devices are rendered conductive during alternate half cycles of said signal voltages when signal currents traverse said system in said one direction and only one of said discharge devices is rendered conductive when signal currents traverse said system in said other direction, a control device including a pair of difierentially related windings respectively included in said circuits, whereby said control device is caused to operate from one position to another when only one of said discharge devices is rendered conductive, and switching means controlled by said control device for controlling the signal current transmission efliciency of at least one of said channels.

11.-A transmission system comprising a pair of channels for transmitting signal currents in difierent directions, a signal current amplifier and a voltage reducing circuit included in one of said ch'annels, said amplifier and said circuit being connected in tandem in the order named in the direction of signal current transmission through said one channel, a second signal current amplifier included in the other of said channels and 'including an input circuit coupled through said voltage reducing circuit to the output circuit of said first-mentioned amplifier, wherebysignal currents traversing said one channel are transmitted through said second amplifier, a circuit network comprising a pair of discharge devices, each of said devices being normally-nonconductive and being characterized by a predetermined break down voltage at which it is rendered conductive, control circuits respectively including said discharge devices and also including means for impressing biasing voltages across said discharge devices which are less in magnitude than said break down voltage, circuit means for impressing signal voltages appearing across said one channel at a point between said first-named amplifier and said voltage reducing circuit upon one of said discharge devices and for impressing signal voltages appearing across said other channel at a point following said sec- .ond amplifier upon the other of said discharge devices, means comprising said voltage reducing circuit and said second amplifier for unbalancing saidsignal voltages when signal currents traverse said other channel, whereby both of said discharge devices are rendered conductive during alternate h'alf cycles of said signal voltages when signal currents traverse said one channel and only one of said discharge devices is rendered conductive during alternate half cycles of said signal voltages when signal ourrents traverse said other channel, switching means normally effective substantially to reduce the signal current transmission efficiency of said other channel, and means operative in response to an unbalancing of said voltage for causing said switching means to increase the signal current transmission efiiciency of said other channel and substantially to reduce the signal current transmission efficiency of said one channel.

12. A transmission system comprising a pair of channels for transmitting signal currents in different directions, a signal current amplifier and a voltage reducing circuit included in one of said channels, said amplifier and said circuit being connected in tandem in the order named in the direction of signal current transmission through said one channel, a. second signal current amplifier included in the other of said channels and including an input circuit coupled through said voltage reducing circuit to the output circuit of said first-mentioned amplifier, whereby signal currents traversing said one channel are transmitted through said'second amplifier, a circuit network comprising a pair of discharge devices, each of said devices being normally nonconductive and being characterized by a predetermined break down voltage at which it is rendered conductive, control circuits respectively including said discharge devices and also including means for impressing biasing voltages across said discharge devices which are less in magnitude than said break down voltage, circuit means for impressing signal voltages appearing acrcss said one channel at a point between said first-named amplifier and said voltage reducing circuit upon one of said discharge devices and for impressing signal voltages appearing across said other channel at a point following said second amplifier upon the other of said discharge devices, means comprising said voltage reducing circuit and said second amplifier for unbalancing said signal voltages when signal currents traverse said other channel, whereby both of said discharge devices are rendered conductive during alternate half cycles of said signal voltages when signal currents traverse 'said' one channel and only one of said discharge devices is rendered conductive during alternate half cycles of said signal voltages when signal currents traverse said other channel, a control device including a pair of dif-- ferentially related windings respectively included in said circuits, whereby said control device is caused to operate from one position to another when only one of said discharge devices is rendered conductive, and switching means controlled by said control device for controlling the signal current transmission efficiency of at least one of said channels.

13. Telephone substation apparatus adapted to be connected to a line and comprising signal transmitting and receiving means, a pair of signal current channels for transmitting incoming signal currents to said receiving means and outgoing signal currents to said line, an amplifier in each of said channels, a voltage reducing circuit included in one of said channels, and means comprising said voltage reducing circuit and the amplifier of the other of said channels for substantially changing the signal current transmission efiiciency of at least one of said channels in response to signal currents traversing one of said channels.

14. Telephone substation apparatus adapted to be connected to a line and comprising signal transmitting and receiving means, a pair of signal current channels for transmitting incoming signal currents to said receiving means and outgoing signal currents to said line, a signal current amplifier and a voltage reducing circuit included in one of said channels, said amplifier and said circuit being connected in tandem in the order named in the direction of signal current transmission through said one channel, a

second signal current amplifier included in the 10 other of said channels, a control circuit coupled to said one channel at a point between said firstnamed amplifier and said voltage reducing cir- ROSWELL H. HERRICK. 

